Layer material for corrosion protection and solar receiver having such a layer material

ABSTRACT

A layer material for corrosion protection and to a solar receiver having such a layer material is provided. The layer material comprises a binding agent consisting of a resin containing at least one of the following substances: oligo- or polysiloxane, silicone resin, silicone, silicate, polyphosphate, and which is dissolved in a solvent, a pigment consisting of zinc microparticles having an average diameter of at least 1 um, wherein a further pigment consisting of titanium oxide or silicon oxide nanoparticles having an average diameter of no more than 100 nm is present in the layer material. The layer advantageously has a self-healing effect in addition to the corrosion protection effect thereof owing to the use of zinc particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/EP2012/066528, having a filing date of Aug. 24, 2012, the entire contents of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to a layer material for corrosion protection. The layer material comprises a binder which consists of a resin and which is dissolved in a solvent. The resin contains at least one of the following substances: oligosiloxane or polysiloxane, silicone resin, silicone, silicate and phosphate. Moreover, the binder contains a pigment consisting of microparticles of zinc having a mean diameter of at least one micrometer. In addition, the following relates to a solar receiver for a solar power plant, comprising an absorber tube made of a metal and an envelope tube made of glass, which surrounds the absorber tube to form an interstice, wherein the interstice is hermetically sealed.

BACKGROUND

A layer material of the type indicated in the introduction is known, for example, from WO 2009/129783 A2. The layer material disclosed therein comprises metal particles, which can be enveloped with inorganic substances. The envelopment is aimed at achieving better results in corrosion protection than with pure zinc layers. On the one hand, the envelopment of the particles also has the effect that the action of the zinc used for corrosion protection is weakened. On the other hand, the envelopments of the metal particles improve the long-term action thereof for corrosion protection, since rapid corrosion of the zinc particles in the case that they are exposed in the layer (for example in instances of damage to the layer) is slowed. In other words, an improved long-term action of the corrosion protection owing to the use of encased metal particles is paid for by the fact that the immediate protective effect in the undamaged layer is weakened.

SUMMARY

An aspect relates a layer material for corrosion protection that comprises zinc particles and ensures both an improved corrosion protection action and improved long-term stability. Moreover, embodiments described herein may open up advantageous fields of use for such a layer material.

Embodiments of the layer material may include a further pigment consisting of nanoparticles of titanium oxide or silicon oxide having a mean diameter of at most 100 nm that is present in the layer material. It is also possible to use a mixture of titanium oxide and silicon oxide. The use of a pigment having nanoscale particles (referred to throughout hereinbelow as nanoparticles) advantageously has the effect that, on the one hand, the envelopment of the microparticles of zinc which are used can be dispensed with. Therefore, the microparticles of zinc can advantageously deploy their full action for corrosion protection. On the other hand, the further pigment/the further pigments consisting of nanoparticles of titanium oxide and/or silicon oxide is/are used instead of the coating of the zinc particles. Since this pigment involves nanoparticles, these can migrate further in the layer material even after the layer has cured. Therefore, the following protective mechanism forms when the layer is used. The lacquer system, which already cures at temperatures of less than 100° C., protects a metallic component against corrosion primarily by virtue of the fact that the zinc pigments (microparticles) serve as cathodic corrosion protection. This presupposes that the metallic component is more noble as a whole. This is the case, for example, for iron and steels, because zinc has a standard potential of −0.76 V and iron has a standard potential of −0.4 V with respect to hydrogen. The cathodic corrosion protection achieved by the zinc pigments can readily be used in temperature ranges of up to 300° C.

If the component, consisting of an iron alloy, is exposed to weathering effects (e.g. moisture, salts, temperature), the zinc pigments undergo corrosive attack and therefore display an action as a sacrificial anode.

The use of nanoscale titanium or silicon oxide (nanoparticles) furthermore stabilizes the layer in the case of damage. This stabilization is brought about by a self-healing process. In the case of damage, the zinc pigments are exposed and thus form a positive surface charge. Without the self-healing process which then follows, the zinc would be degraded very quickly on account of the corrosive attack and would then no longer be able to ensure corrosion protection. However, the nanoparticles of titanium oxide and/or silicon oxide carry a negative surface charge and therefore migrate toward the damaged site. As a result of this, the damaged site is covered and a layer is also formed on the exposed zinc pigments, this replacing the matrix of the layer material at this site. As a result of this, the degradation of the zinc pigments at the damaged site is slowed once again, as a result of which the layer can be referred to as self-healed in terms of its function.

It has now been found that the effect described can be optimized if the microparticles of zinc do not exceed a mean diameter of at most 100 μm. The production of the layer material is furthermore advantageously simplified if the nanoparticles of titanium oxide or silicon oxide do not fall below a mean diameter of at least 100 nm.

Moreover, according to a further configuration, it is possible to add a further pigment consisting of aluminum oxide, zirconium oxide or silicon oxide, with a mean diameter of these particles of at least 1 μm. These further pigments serve as a filler and improve the thermal stability of the coating. The latter can then also withstand higher temperatures, it being necessary in any case to take into consideration the melting point of zinc, which is 415° C.

Moreover, it is possible to add further auxiliary substances. Hydrophobic silica can serve as a thixotropic agent and an anti-sedimentation aid (dispersant) for the solid additives. In addition, with its hydrophobic properties, it also improves the protective action of the layer to be produced. It is possible to add 1 to 3% by mass of hydrophobic silica. In addition to the substances already mentioned, aluminum oxide, zirconium oxide and silicon oxide, carbon nanotubes and boron nitrite nanotubes are also to be specified in this context.

Embodiments may also relate to a solar receiver for a solar power plant, comprising an absorber tube made of metal. Said absorber tube is accommodated in an envelope tube made of glass, such that an interstice forms between the two tubes. The interstice is hermetically sealed. The externally accessible part of the solar receiver is equipped with a layer (outside the transparent envelope tube) made of a cured layer material, which is configured in the manner already described above. Thus, specifically, the layer material comprises a matrix consisting of a resin which contains the above-mentioned substances. Moreover, microparticles of zinc are incorporated in the matrix as a pigment and nanoparticles of titanium oxide and/or silicon oxide are incorporated in the matrix as a further pigment. Because the layer is cured, the solvent is evaporated completely or at least predominantly from the resin, as a result of which the cured layer material is formed.

According to an embodiment of the solar receiver, it is moreover provided that the hermetic seal of the interstice is ensured by a weld seam. The weld seam is equipped on the externally accessible part thereof with the layer made of the cured layer material. This advantageously has the effect that the spacer ring is reliably protected against corrosion. The externally accessible part of the spacer ring adjoins the envelope tube at one end and the absorber tube at the other end. These connection sites can likewise be spanned by the layer material. As a result of this, the sealing sites for the hermetic seal can also be reliably protected against corrosion.

BRIEF DESCRIPTION

Further details of the invention will be described hereinbelow on the basis of the drawing. Identical or corresponding elements of the drawing are in each case provided with the same reference signs, and are explained repeatedly only where there are differences between the individual figures, in which:

FIG. 1 depicts an exemplary embodiment of the layer material in a schematic section;

FIG. 2 depicts an exemplary embodiment of the layer material in a schematic section; and

FIG. 3 depicts an exemplary embodiment of the solar receiver in a longitudinal section.

DETAILED DESCRIPTION

As shown in FIG. 1, a layer 11 is applied to a component 12. The layer has a matrix consisting of a binder 13, in which microparticles 14 of zinc are incorporated. Moreover, nanoparticles 15 of titanium oxide and/or silicon oxide are distributed uniformly in the binder 13. In addition, particles 16 of a filler, e.g. aluminum oxide, can also be provided in the matrix.

FIG. 2 shows a layer structure comparable to FIG. 1. However, in this embodiment, no particles of a filler are provided. FIG. 2 shows, however, how damage 17 in the form of a crack changes the layer structure. It becomes clear that individual microparticles 14 a of zinc are exposed in the crack surface 18. This results in an increased rate of corrosion of the exposed microparticles 14 a, and therefore a positive surface charge forms locally on the crack surface 18. Because the nanoparticles 15 of oxide can migrate in the layer matrix formed by the binder 13 on account of their small size, they migrate within a diffusion zone 25 (with dot-dash hatching), on account of their negative surface charge, to the crack surface 18, where they lead to a concentration. In particular, the exposed microparticles 14 a of zinc are also covered, and therefore the activity thereof is reduced again and levels out at a level which is comparable to that of the microparticles 14 of zinc which are completely incorporated in the binder 13. Therefore, with respect to the corrosion protection properties of the layer 11, it is possible to refer to self healing. Although the damage 17 is still present, the corrosion in this region of the layer is not accelerated, and therefore the component 12 remains protected against a corrosive attack at this point too.

FIG. 3 shows a solar receiver 19, which is made up of an absorber tube 20, an envelope tube 21 and a spacer ring 22. The spacer ring 22 leads to a central mounting of the absorber tube 20 in the envelope tube 21, as a result of which an interstice 23 forms. This interstice 23 is considered to be insulation.

In order that the different coefficients of expansion of the envelope tube 21 and of the absorber tube 20 can be compensated for, the spacer ring 22 is mounted displaceably on the absorber tube 20 via a gap 30 (for example a clearance fit) (shown in exaggerated form). In order to obtain hermetic sealing of the inner space 23, bellows 31 made of sheet metal are provided to compensate for the axial movements of the spacer ring 22 on the absorber tube 20. These bellows are supported by way of a base ring 32 on the absorber tube 20, where the latter is fixed by a welded connection 33. The connection between bellows 31 and base ring 32 can be a pressed connection 34. The connection between the spacer ring 22 and the bellows 31 is made by way of an intermediate ring 35, which is likewise connected to the bellows 31 by way of a pressed connection. The connection ring 35 and also the spacer ring 22 are connected by a welded connection 36 at their joint. This provides the hermetic seal between the spacer ring 22 and the intermediate ring 35.

In order to protect the region of the hermetic seal of the interstice 23 by the weld seams 33, 36, a layer 24 is applied in these regions, said layer having for example the form as shown in FIG. 1 in a manner not shown in more detail. This brings about corrosion protection of the externally accessible parts of the weld seams 33, 36. The adjoining parts can likewise be coated (not shown).

Methyl silicone resin solutions or methyl phenyl silicone resin solutions can be used, for example, as the binder. Trade names for these substances are, for example, Silres® REN50, REN60 or REN80 from Wacker. Another possibility consists in the use of hydrophobic silica. This can be purchased, for example, under the trade name HDK H13L or HDK H15 from Wacker. These substances are dissolved in butanol, xylene or a mixture of these solvents. When these solvents are used, room temperature is already sufficient for drying. If silica is added to the layer material in a concentration of 1 to 3% by weight, it displays its action both as a thixotropic agent and as a dispersant for the solid additives. In addition, it gives the surface of the layer to be produced hydrophobic properties. Said layer material can be processed, for example, as lacquer. The particles used are processed to form a lacquer system which contains said solvents and binders and which is processed as a dispersion of the particles. If use is made of a special methyl polysiloxane resin (for example Silres® MSF 100 from Wacker), the lacquer can cure, using catalysts at room temperature and with a relative atmospheric humidity of 50%, to such an extent over half an hour that it is firm to the touch. Acids, bases, tin, zinc, titanium and zirconium compounds can be used as the catalyst. Since the microparticles consist of zinc anyway, the presence of a catalyst is ensured in the lacquer system.

The lacquer system can be applied by spraying, immersion or painting. Spraying using a compressed air gun is advantageous for the production of anti-corrosion layers on the application of solar receivers. This can advantageously be effected at the construction site. As a result of this, it is also possible to readily carry out repairs to plants which have already been installed. On the other hand, it is also possible for a compressed air gun to be readily integrated into the course of the procedure for the initial installation of the solar receiver. In this case, use can be made, for example, of pneumatically controlled automatic guns. 

1. A layer material for corrosion protection, comprising: a binder having a resin containing at least one of the following substances: oligosiloxane or polysiloxane, silicone resin, silicone, silicate, polyphosphate, and which is dissolved in a solvent; and a pigment including microparticles of zinc having a mean diameter of at least 1 μm; wherein a first further pigment including nanoparticles of titanium oxide or silicon oxide having a mean diameter of at most 100 nm is present in the layer material.
 2. The layer material as claimed in claim 1, wherein the microparticles of zinc have a mean diameter of at most 100 μm.
 3. The layer material as claimed in claim 1, wherein the nanoparticles of titanium oxide or silicon oxide have a mean diameter of at least 10 nm.
 4. The layer material as claimed in claim 1, wherein the layer material contains a second further pigment including aluminum oxide, zirconium oxide or silicon oxide, with a mean diameter of the particles of at least 1 μm, as filler.
 5. A solar receiver for a solar power plant, comprising: an absorber tube made of a metal; and an envelope tube made of glass, which surrounds the absorber tube to form an interstice, wherein the interstice is hermetically sealed, wherein an externally accessible part of the solar receiver outside the envelope tube is equipped at least partially with a layer made of a cured layer material.
 6. The solar receiver as claimed in claim 5, wherein the hermetic seal of the interstice is ensured by a weld seam, wherein the weld seam is equipped on the externally accessible part thereof with the layer made of the cured layer material.
 7. The solar receiver as claimed in claim 5, wherein the cured layer material on the envelope tube comprises: a binder of a resin containing at least one of the following substances: oligosiloxane or polysiloxane, silicone resin, silicone, silicate, polyphosphate, and which is dissolved in a solvent; and a first pigment including microparticles of zinc having a mean diameter of at least 1 μm, wherein a second pigment including nanoparticles of titanium oxide or silicon oxide having a mean diameter of at most 100 nm is present in the cured layer material.
 8. The solar receiver as claimed in claim 7, wherein the microparticles of zinc have a mean diameter of at most 100 μm.
 9. The solar receiver as claimed in claim 7, wherein the nanoparticles of titanium oxide or silicon oxide have a mean diameter of at least 10 nm.
 10. The solar receiver as claimed in claim 7, wherein the cured layer material contains a third pigment including aluminum oxide, zirconium oxide or silicon oxide, with a mean diameter of the particles of at least 1 μm, as filler.
 11. The solar receiver as claimed in claim 6, wherein the cured layer material on the weld seam comprises: a binder of a resin containing at least one of the following substances: oligosiloxane or polysiloxane, silicone resin, silicone, silicate, polyphosphate, and which is dissolved in a solvent; and a pigment including microparticles of zinc having a mean diameter of at least 1 μm, wherein a first further pigment including nanoparticles of titanium oxide or silicon oxide having a mean diameter of at most 100 nm is present in the cured layer material.
 12. The solar receiver as claimed in claim 11, wherein the microparticles of zinc have a mean diameter of at most 100 μm.
 13. The solar receiver as claimed in claim 11, wherein the nanoparticles of titanium oxide or silicon oxide have a mean diameter of at least 10 nm.
 14. The solar receiver as claimed in claim 11, wherein the cured layer material contains a second further pigment including aluminum oxide, zirconium oxide or silicon oxide, with a mean diameter of these particles of at least 1 μm, as filler. 